The present invention relates to the packaging field, and more particularly to a laminated thermoplastic film which can be used to produce a heat-sealable, heat-shrinkable bag having a polyester external surface.
The use of heat-shrinkable thermoplastic films is well known to the packaging industry. For example, poultry products are typically sealed within bags made from such films and heated, thus shrinking the bag until it fits tightly about the product.
One type of thermoplastic material currently used to form such bags is monolayer polyester film. Polyester bags have many advantages. For instance, they provide strength and protection through tight adhesion to the product. Also, printing onto treated polyester tends to be somewhat more stable than onto other thermoplastic materials. However, a problem exists in that polyester is not heat-sealable except at exceptionally high temperatures, and as a result such bags must be sealed with adhesive. Bags sealed with adhesive are not as strong in the seal area as than heat sealed bags, cannot be closed on the open end by existing heat seal equipment and cannot be printed so as to lock or protect the printing.
A second type of heat-shrinkable bag currently in use is made of co-extruded, heat-shrinkable, thermoplastic films, such as polyolefin. For example, U.S. Pat. No. 3,299,194 to Golike and U.S. Pat. No. 3,663,662 to Golike et al. disclose shrink films of oriented polyethylene and various copolymers of ethylene. U.S. Pat. No. 4,597,920 to Golike teaches a shrink films which is a copolymer of ethylene with at least one C.sub.8 -C.sub.18 .alpha.-olefin. Methods for producing multi-layer thermoplastic film are provided in U.S. Pat. No. 2,855,517 to Rainer et al., U.S. Pat. No. 3,022,543 to Baird, U.S. Pat. No. 3,754,063 to Schirmer, and U.S. Pat. No. 3,981,008 to D'Entremont.
Coextruded films, such as polyolefins, are useful in producing heat-shrinkable bags because they are heat-sealable and therefore can be produced on existing heat seal equipment economically. They maintain good physical contact with a packaged product after heat shrinking, and thereby retain juices within packaged meats, but not as well as the laminated shrink bags. However, coextruded films have different mechanical properties, such as tensile strength and modulus, and therefore bags made from these films are more apt to tear or otherwise become physically damaged during handling than a multilayer lamination. Another important disadvantage is that you cannot reverse print or lock the print between the film layers, whether by surface or reverse print, with coextruded films, making them exposed to abuse, abrasion, and removal of the print by physical or chemical action.
Therefore, there exists a need for a thermoplastic film which combines the advantages and eliminates the disadvantages associated with monolayer polyester and coextruded films.
More particularly, there exists a need for a film which is strong, which is heat-sealable, and which eliminates the problem associated with surface printing.